Is this phosphorus deficiency?

Your NL plant is suffering from a general chronic under feeding, and the damage looks like a mix of potassium and calcium deficiencies. The Runtz plant seems fine. All plants have differing needs, what might be good for one may not necessarily be good for another. Feeding the different strains the same nutrients might not work, as has happened in your case. It should have been obvious weeks ago that the Northern Lights plant just wasn't getting enough fertilizers to maintain health, however what you were giving was totally adequate for the Runtz plant. This could have been easily kept in check by giving the NL plant some extra feeding or slightly stronger feeds than the Runtz plant. To fix your NL plant you will have to feed it more than your Runtz plant, a fairly simple procedure. She definitely needs more cal/mag and extra P/K booster. Do this and she should pick up. Giving both plants a dose or two of soluble trace elements (available from any garden center) will also boost health. Giving the NL plant some seaweed tonic/extract will also help it recover faster and boosts its metabolism too. Hope this helps, Organoman.